1. Field of the Invention
This invention relates to magnetic disk drive storage systems having moveable magnetic transducers and more particularly to filter circuits that process the position error signal developed by the electronic servo circuitry which positions the magnetic transducers.
2. Description of the Prior Art
Many magnetic data recording systems utilize a recording disk with a coating of magnetic material that is selectively magnetized to record digital information. In order to record information at a high density, the magnetic disk is normally rotated at high speeds by an electric motor and has a large number of closely spaced "tracks" on which information is recorded on its surfaces. In this type of magnetic disk recording system, or magnetic disk memory, magnetic transducers of "heads" are provided to read information from or to write information on the magnetic disk surfaces.
In many systems a head is provided for each data track on the disk surface. In other systems, a single head or sets of heads are used to read and write all data tracks. These heads are moved over the disk surface by means of a drive carriage which is in turn controlled by a servomechanism. The servomechanism moves the drive carriage and by using servo signals, which are recorded on one surface of the magnetic disk, positions each head over a single data track to process the information located in that track.
There are several methods for recording the servo information on the disk surfaces. In one common implementation one disk surface called a "servo surface" is entirely dedicated to servo information. In this type of system a dedicated head called a "servo head" is used to read the servo information from the servo surface. The servo head is linked mechanically to other heads which read or write data information on other data tracks. During manufacture of the disk drive, the data heads are aligned relative to the servo head. During subsequent operation the servo circuitry moves the entire head unit, including the servo and data heads, until the servo head is in the proper location with relation to the servo data. Since the servo and data heads are in alignment, the data heads are thereby properly positioned.
The servo data which is recorded on the servo surface is typically a number of repetitive pulses which occur at a fixed frequency as the disk moves past the servo head. The servo information is arranged to change as the servo head is moved across tracks. Thus, the information can also be used to count the number of tracks over which the servo head was moved; allowing the attached data heads to be moved from one data track to another data track under control of the servomechanism. When the servo data is recorded at the proper frequency, track counting is reliable and the system operates properly.
In some cases, however, it is necessary to increase the number of data tracks per disk surface in order to increase the storage capacity of the disk. The number of servo tracks will also increase as there is generally a one-to-one correspondence between the number of servo and data tracks. Often when the track "density" is increased, it is not possible to increase the servo data frequency proportionally to the increase in track density without a major modification of the servo writing equipment and without redesigning the servo channel circuitry in the disk drive. In addition, increasing the servo density also increases the probability of significant effects resulting from noise pickup from an adjacent data head in the process of writing. These effects occur because as the servo data frequency increases it becomes closer to the frequency of the data being written and data frequency pickup cannot be filtered out from the servo information. Therefore, in many cases the servo frequency must remain the same when the track density is increased.
When the track density is increased and the servo frequency remains the same, the reliability of the track counting circuitry is decreased due to several factors. One factor is a decrease in the signal to noise ratio of the position error signal which is used by the servo system to properly locate the heads. This occurs because noise in the servo circuitry generally remains constant as the track density is increased. However, a narrower servo head must be used in conjunction with the higher track density and therefore, the head output amplitude is decreased and the signal to noise ratio decreases proportionally. In addition, since the narrower head "sees" a smaller portion of the disk surface, defects in the magnetic coating of the disk contribute more noise to the signal.
A poor signal-to-noise ratio has serious effects, particularly at low carriage velocities due to the fact that the position error signal is fed into comparators which determine a threshold voltage used in track counting. If a sufficiently noisy signal is applied to the track counting comparators, the hysteresis of the comparator may be exceeded by the noise voltage and multiple track crossings may be indicated even though in fact only one track has been crossed. Since the velocity at which the head carriage moves is normally determined by the number of remaining tracks to go to the destination, at low velocities an error in track count can cause a significant increase in positioning time.
Another problem which results from increased track density is error caused by missing servo information. Servo information may not be recorded properly on the disk surface, and in addition in some systems the absence of servo information is used to index a particular section of the disk. At high track densities and high carriage velocities, only a few pulses of servo information may be "seen" by the servo head as the head crosses each track. If some of these pulses are missing, the position error signal developed from the servo pulses may not exceed the threshold necessary for track counting and an improper track count will result.
One method which has been used to attempt to solve the noise problem is to pass the servo information through a low pass filter. This has the advantage of removing noise which causes track counting errors at low carriage velocities. However, as the carriage velocity increases, and greater numbers of tracks are crossed per unit of time, the frequency of the position error signal derived from the servo information also increases, and therefore the output of the low pass filter also decreases. Once the signal amplitude falls below the threshold of the comparator used for track counting, no track counting occurs and the head positioning carriage will overshoot its destination.